When Franklin arrived at King's College in 1951, J. T.
Randall's biophysics program had been in existence for only
five years. Like many researchers of that era, Randall,
the inventor of radar, was interested in using physics to
determine the structure of large molecules so as to
understand how they functioned in larger organisms. The
molecules involved in heredity were of special interest.
Though Oswald T. Avery had proposed that DNA was the key
chemical substance of heredity, many scientists believed
that proteins were more likely candidates. Randall's
program aimed to investigate all aspects of "the physical
factors affecting mitosis and cell division, by direct and
indirect methods."

Franklin's fellowship proposal called for her to work on x-ray
diffraction studies of proteins in solution. However,
there was a shift in research priorities after Maurice
Wilkins, the assistant director of Randall's lab, began
working with an unusually pure sample of DNA obtained from
Rudolf Signer. Excited about the possibilities, Wilkins
suggested to Randall that Franklin's expertise might be
better applied to this promising DNA research. Randall
agreed; he wrote to Franklin in November 1950, explaining
the change of plan, and stated that she and graduate
student Raymond Gosling would be the only staff doing
crystallographic studies of DNA. Randall did not mention
Wilkins' serious interest in DNA, nor did he tell Wilkins
the details of the letter. These omissions soon generated
misunderstandings between Wilkins and Franklin--Franklin
assumed that the x-ray diffraction studies of DNA would be
her project alone; Wilkins assumed that she was joining the
loosely organized research team ("Randall's Circus") at the
biophysics lab, as the expert on crystallography. When
Wilkins continued working on DNA and suggested that he and
Franklin collaborate, she resented what she regarded as
interference. The situation was exacerbated by differences
in personality. Franklin had always been direct, honest,
and unafraid of argument; indeed, with her colleagues in
Paris, she learned to enjoy "a good row." As friends and
family often noted, she could be kind, generous, and fun,
but she did not suffer fools gladly. Wilkins had a
gentler, more reticent personality, and found Franklin's
manner intimidating at times. Besides this, Franklin had
been reluctant to return to England and to what she viewed
as a stultifying intellectual and social climate. Though
Randall's staff included a number of women, King's College,
founded as a seminary for Anglican clergymen, was still a
largely male preserve, maintaining a separate common room
for men, and another which was open to both men and women.
Franklin sorely missed the egalitarian atmosphere of Paris,
and may have kept her distance from the team in London
simply out of homesickness for "le labo." As a result,
Franklin and Wilkins did not collaborate to any great
extent as the work on DNA went forward.

The material that Franklin was to study was calf thymus
DNA, extracted and purified by Rudolf Signer of Berne, and
given to several researchers, including Wilkins, at a
conference in May 1950. Signer had used an extremely gentle
technique that avoided breaking up the large molecules.
Preparing some of it for optical studies with a reflecting
microscope, Wilkins discovered that this DNA could be drawn
out into very thin, uniform fibers like a spider web. This
suggested to him a regular structure that might yield an x-ray
diffraction pattern, so he and Gosling improvised an
apparatus and took photos of the samples. Varying the
relative humidity gave them several different results, some
showing a disordered pattern and others showing a
crystalline pattern. Meanwhile further clues about the
possible shape of DNA had come from infra-red spectroscopy
studies at King's and the work of Sven Furberg at Birkbeck
College (in Furberg's models, the DNA was in helical form,
with the sugars perpendicular to the bases and the bases on
the inside). In May of 1951, Wilkins gave a paper at a
meeting in Naples in which he outlined this and other work
being done at King's. (One of those attending the
presentation was James Watson, who was intrigued by the
findings.) By July, Wilkins was considering that the DNA
molecule might be helical, like the polypeptide chain of
the protein alpha-helix recently discovered by Linus
Pauling. He asked Alec Stokes, a theoretical physicist at
King's, to determine mathematically what sort of x-ray
diffraction pattern a helix would produce; Stokes rapidly
produced a pattern with a strong central "X" much like the
one Wilkins and Gosling had made. When Wilkins presented
these ideas at a small meeting in Cambridge that month,
however, Franklin quietly told him afterwards to leave the
diffraction work to her and (as Wilkins later recalled) to
go back to his microscopes. Though shaken and confused by
this, Wilkins still expected that their relations might
improve. In September, he returned from several summer
conferences excited about what he had learned, and with a
vial of DNA from Erwin Chargaff's lab, which he hoped would
help expand the DNA work.

In the meantime Franklin had assembled her new x-ray
equipment and Phillips micro-camera and settled down to
work. She solved the problem of keeping the humidity
constant inside the camera by bubbling hydrogen through
salt solutions to the desired humidity levels. Working with
single DNA fibers at very high humidity, she discovered
that there were two forms of DNA: the familiar "dry"
crystalline form ("A") and a longer, thinner, heavily
hydrated "paracrystalline" form, which she and Gosling
called the "B" or "wet" form. Samples could shift from one
form to the other if humidity levels changed. (The
existence of two distinct DNA forms explained why earlier
attempts at diffraction pictures, such as William
Astbury's, had been so fuzzy: the samples contained both
forms.) Franklin noted, "Either the structure is a big
helix or a smaller helix consisting of several chains. The
phosphates [of the sugar that forms the backbone of the
molecule] are on the outside so that phosphate-phosphate
inter-helical bonds are disrupted by water." The
hydrophilic phosphates caused the molecule to soak up water
and lengthen. The wet DNA produced a sharp diffraction
picture that resembled the pattern Stokes had predicted.
Very pleased when he heard of this, Wilkins suggested that
he and Stokes collaborate with Franklin. She angrily
refused, and their relations became quite strained. At
Randall's prompting, they reached a compromise: Franklin
would work on the A form, using the Signer DNA, and Wilkins
would work on the B form, using the Chargaff DNA. The
Chargaff sample, which had been degraded during extraction,
turned out to be unsuitable for diffraction studies and
Wilkins was unable to make much progress.

Franklin and Gosling continued to photograph and analyze
samples of DNA in the A form. The A form presented a
different picture, far from clearly helical. Though it was
hard to think how DNA could be helical in one form but not
in another, the mathematical analysis that Franklin and
Gosling carried out did not support a helical structure.
Franklin's notebooks from 1951-1952 show that she thought a
helix possible, but her data on the A form did not yet
confirm it, and she would not theorize in advance of the
evidence. To her, the proper approach was to gather data
first and then build models from them, not the other way
round.

James Watson and Francis Crick at Cambridge University's
Cavendish Laboratory had meanwhile been taking the other
way round, attempting to build a model of DNA based on what
was already known, and on what current research, including
that at King's, suggested. They believed that DNA was
helical, and in November 1951 built a model of a three-helix
molecule with the phosphates on the inside. The
biophysics staff at King's, including Franklin, were
invited to Cambridge to see it. Franklin immediately
noted, correctly, that such a configuration would not hold
together. In response, the director of the Cavendish,
Lawrence Bragg, ordered Watson and Crick to cease work on
DNA and leave it to King's. They did so, going so far as to
send their model parts down to the King's staff, though
Franklin had little use for them.

During 1952 Franklin took and analyzed ever-sharper
photographs of the A form but still didn't see a helix.
Her mathematical analysis, the Patterson function, though
consistent with a helix in some ways, seemed to show a
structural repeat of a dimension that would make a helical
folding of the molecule impossible. In July, she and
Gosling posted a prank notice of the "death of DNA helix
(crystalline)." Franklin felt obliged to consider non-helical
structures for the A form. She also believed that
the A form, being more crystalline, would yield more
precise information. Because DNA could shift between the
two forms, any model would need to account for both forms.
Yet she at no time argued that the B form was not helical,
and this was reflected in the report that was made to the
Medical Research Council (which funded the Biophysics Unit)
late that year.

Watson and Crick had not stopped thinking about DNA, and
they were in regular communication with Wilkins, eager to
learn whatever they could of the progress at King's. In
January 1953, spurred by Linus Pauling's publication of a
3-helix model (similar to the one they made in 1951), they
resumed work on their DNA model, determined to get it right
before Pauling or someone else did. Two pieces of evidence
from Franklin's work were crucial to their correct model:
first, a very clear photo of the B form taken in May 1952
labeled "51" which Gosling had given to Wilkins as part of
his graduate research work, and which Wilkins showed to
Watson without Franklin's knowledge; and second, the MRC
report, given to Watson and Crick by Max Perutz, a member
of the MRC committee that reviewed the work at Randall's
lab. The report contained details of Franklin's work (as
yet unpublished), including her identification of the unit
cell as belonging to the crystal space group called face-centered
monoclinic C2. The photo confirmed the helical
pattern, and the unit cell type told Crick, a physicist
with more theoretical crystallography expertise than
Franklin, that the helices ran in opposite directions. By
early March, they had their model.

Franklin, still unhappy at King's, had arranged to transfer
to J. D. Bernal's lab at Birkbeck College, and was hurrying
to finish writing up her work on the A form before leaving.
She was unaware of the "race for the double helix" that was
in process. In February 1953, however, she looked again at
photo #51 and began analyzing it. Several days later she
concluded that both A and B forms were two-chain helices,
although she had not resolved the configuration of the
bases inside. She and Gosling drafted an article on the
likely molecular structure by mid-March. This appeared, in
expanded and modified form, with Watson and Crick's
announcement in Nature on April 25,
but the draft was done
before they had heard about the Watson-Crick model. When
Franklin saw the model, she readily accepted it. She and
Gosling soon tested the model against their diffraction
data for both A and B forms and found that it fit very
well. They published an article on these findings in July
1953. Though she subsequently enjoyed very cordial
relationships with both Crick and Watson, they never fully
acknowledged the important role played by her x-ray
diffraction data in their discovery. Shortly after Watson
and Crick's announcement, Franklin left the King's lab and
DNA work, and moved to Birkbeck College to study virus
structure.